<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<script>
    <!--    先声明-->
    var age;
    // 再使用
    age = 18;
    //    输出结果
    console.log(age);

    var myname = "gtt";
    console.log(myname)
</script>
</body>
</html>